1. Field of the Invention
The present invention relates to bonded substrate structures and to a method for fabricating bonded substrate structures. For example, the invention relates to bonded substrate structures with a plurality of substrates being bonded together, including those with a glass substrate bonded to a semiconductor substrate and those with a plurality of different semiconductor substrates bonded together, and relates to a method for fabricating such bonded substrate structures. In particular, the technique of the invention is suitable to bonded substrate structures with a glass substrate bonded to a semiconductor substrate, and to a bonding method for fabricating such bonded substrate structures. The bonded substrate structures and the bonding method for them of the invention are especially useful in the field of MEMS (micro-electro-mechanical systems).
2. Description of the Related Art
In the field of semiconductors, much used are bonded substrate structures with a plurality of substrates being bonded together, including, for example, those with a glass substrate bonded to a semiconductor substrate and those with a plurality of different semiconductor substrates bonded together. For example, known is a bonded substrate structure with a glass substrate bonded to a semiconductor substrate, in which various devices are housed in device-housing recesses formed in the glass substrate and the recesses are covered and sealed with the semiconductor substrate. Another bonded substrate structure is also known, in which movable devices such as mirrors, lenses and the like are fitted to the upper layer substrate and devices for driving the movable devices are fitted to the lower layer substrate.
A micro-mirror device is one example of a bonded substrate structure with a semiconductor substrate bonded to a glass substrate. FIGS. 1A and 1B show an ordinary micro-mirror device structure. In such a micro-mirror device, the mirror angle is variable, and a voltage is applied to the driving electrode housed in the device-housing recess formed in the glass substrate thereby to drive the mirror movably fitted to the semiconductor substrate.
FIGS. 1A and 1B are referred to, which show the structure of such a micro-mirror device. Precisely, FIG. 1A is an outline view of a micro-mirror device; and FIG. 1B is an exploded view thereof in which the upper semiconductor substrate is separated from the lower glass substrate. As in FIGS. 1A and 1B, the micro-mirror device comprises a semiconductor substrate 101 and a glass substrate 102. As therein, a mirror 104 is fitted to the semiconductor substrate 101. The mirror 104 is supported by beams 105 at the facing two corners, and its angle is variable around the pivotal axis of each beam 105. The electrode for driving the mirror 104 is formed in the device-housing recess of the glass substrate 102. Various devices are housed in the device-housing recess 103. After the devices to be in the device-housing recess 103 have been formed, the glass substrate 102 is bonded to the semiconductor substrate 101.
FIGS. 2A to 2E show a process of fabricating such micro-mirror devices. In the process illustrated, a plurality of micro-mirror devices are formed on one laminate substrate composed of a semiconductor substrate and a glass substrate and having a size of 20 mmxc3x9720 mm, and these are finally cut into individual devices.
As in FIG. 2A, a glass substrate 202 with a mirror-driving electrode and other devices having been formed in each device-housing recess 203 is bonded to a semiconductor substrate 201 of silicon. For bonding them, for example, the two substrates are subjected to anodic bonding at 300 to 400xc2x0 C. and at a voltage falling between 0.5 and 1.0 kV.
After the two substrates are thus bonded together, an Al film 204 for mirrors is formed on the Si substrate 201 through vapor deposition, as in FIG. 2B. Next, a resist pattern 206 for mirrors is formed, as in FIG. 2C. This is put into a solution of, for example, phosphoric acid, by which the Al film 204 except the area below the resist pattern is removed to give resist-coated mirrors, as in FIG. 2D. Next, the resist film is removed to form mirrors, as in FIG. 2E. The process does not interfere with the devices in the glass substrate 202.
In order that the mirror in each device thus formed is supported by two beams, as in FIGS. 1A and 1B, the area around the mirror must be etched away, for example, through dry etching. Dry etching shall be effected in a vacuum of from a few mTorr to tens mTorr.
The glass substrate 202 is bonded to the Si substrate 201, for example, through anodic bonding as in the above. In case where they are bonded in that manner at an atmospheric pressure, a vapor of around 0.4 atmospheres will be sealed in the device-housing recesses 203 in the glass substrate 202. Accordingly, when the area around the mirrors is etched in dry in a vacuum falling between a few mTorr and tens mTorr, the gas remaining in the sealed device-housing recesses 203 will jet out at a high speed immediately after the Si substrate around the mirrors has been removed to give through-grooves reaching the device-housing recesses 203. As a result, the fine structures formed in the device-housing recesses 203 and even the mirror-supporting beams will be broken or damaged.
To prevent the gas from jetting out of the sealed device-housing space in the dry-etching process as above, employable is a method of forming openings running outside through any one of the glass substrate and the semiconductor substrate in the direction of their depth before the two substrates are bonded together. In the bonded substrate structure formed in the method, gas is not sealed in the device-housing space but could pass through the openings formed. Another method employable for that purpose comprises bonding the two substrates in vacuum. In this, the sealed device-housing recesses 203 in the glass substrate 202 are kept in vacuum before the substrate 202 is bonded to the other semiconductor substrate.
FIGS. 3A to 3C show a process for fabricating a micro-mirror device, in which openings are formed through the semiconductor substrate so as to prevent gas ejection from the sealed device-housing recesses. These is to typically illustrate the process of fabricating one micro-mirror device.
In the process of FIGS. 3A to 3C, a glass substrate 302 with a device-housing recess 303 formed therein is bonded to a semiconductor substrate 301. For example, they are bonded together through anodic bonding at 300 to 400xc2x0 C. under atmospheric pressure, like in the manner mentioned above. As in FIG. 3A, openings 309 are first formed in the region 308 to be etched (this is surrounded by the dotted line), around the mirror-forming region 307.
Next, the glass substrate 302 is bonded to the semiconductor substrate 302 through anodic bonding, as in FIG. 3B.
Finally, the region 308 to be etched (surrounded by the dotted line in FIG. 3A) is etched in dry to finish the structure of FIG. 3C in which the mirror 304 is supported by beams 305. The dry etching is effected in a vacuum falling between a few mTorr and tens mTorr. In this process, since the device-housing space 303 is open to the outside through the openings 309, the pressure inside it could be kept the same as that outside it with no rapid pressure change during the etching step.
However, the method for preventing rapid pressure change in the device-housing space by forming openings running outside through any one of the glass substrate and the semiconductor substrate in the direction of their depth before the two substrates are bonded together, as in FIGS. 3A to 3C, requires the additional step of forming the openings. For example, it additionally requires resist patterning and dry-etching for forming the openings. As being so complicated, the method is therefore unfavorable. In addition, the strength of the substrate thus having such a plurality of openings therein is lowered, and the substrate will be broken or damaged in the bonding step or in the step before or after the bonding step. On the other hand, if the cross-sectional area of the openings is reduced so that it does not occupy any superfluous region and that the openings do not lower the strength of the substrate, the aspect ratio of the openings shall be large and it is difficult to form the openings having such a large aspect ratio.
Regarding the other method of bonding the semiconductor substrate and the glass substrate to each other in vacuum, the step of increasing the degree of vacuum in every closed device-housing space in the center region of one wafer having a number of devices therein will take a lot of time, thereby lowering the production efficiency.
In consideration of the problems with the related art techniques noted above, the present invention is to provide bonded substrate structures with a plurality of substrates being bonded together and a method for fabricating such bonded substrate structures with no rapid pressure change in the step of forming a device-housing space in the structures. Specifically, in the method of fabricating bonded substrate structures of the invention, the device in the structures is not broken or damaged in any stage of bonding the substrates or etching them to form a through-groove therein, and the method itself is not complicated.
The bonded substrate structures and the method for fabricating them of the invention are especially effective in the field of MEMS (micro-electro-mechanical systems).
The invention has been made in consideration of the problems noted above, and its first aspect is to provide a bonded substrate structure with a plurality of substrates being bonded together, in which at least one bonded substrate is so constituted that its bonded surface has a through-groove running inwardly from its peripheral edge.
In one embodiment of the bonded substrate structure of the invention, the through-groove reaches a device-housing recess formed in at least one bonded substrate and is open thereto so that the atmosphere outside the bonded substrate structure can be kept nearly the same as the atmosphere inside the device-housing recess via the through-groove.
In another embodiment of the bonded substrate structure of the invention, the bonded surface of at least one substrate has a device-housing recess and the through-groove, and the device-housing recess and the through-groove have nearly the same depth.
In still another embodiment of the bonded substrate structure of the invention, at least one bonded substrate has a plurality of device-housing recesses corresponding to a plurality of devices, the through-groove is configured to comprise a plurality of through-grooves connecting the device-housing recesses to each other, and the through-grooves formed inside the bonded substrate and remoter from the peripheral edge thereof are made to have a larger conductance for the fluid running therethrough than those formed adjacent to the peripheral edge of the bonded substrate.
In still another embodiment of the bonded substrate structure of the invention, the cross-sectional area of the through-grooves formed inside the bonded substrate is made larger than that of the through-grooves formed adjacent to the peripheral edge of the bonded substrate to thereby adjust the conductance of the through-grooves for the fluid running through them.
In still another embodiment of the bonded substrate structure of the invention, the number of the through-grooves formed inside the bonded substrate is made larger than that of the through-grooves formed adjacent to the peripheral edge of the bonded substrate to thereby adjust the conductance of the through-grooves for the fluid running through them.
In still another embodiment of the bonded substrate structure of the invention, the width of the through-grooves formed inside the bonded substrate is made larger than that of the through-grooves formed adjacent to the peripheral edge of the bonded substrate to thereby adjust the conductance of the through-grooves for the fluid running through them.
In still another embodiment of the bonded substrate structure of the invention, the depth of the through-grooves formed inside the bonded substrate is made larger than that of the through-grooves formed adjacent to the peripheral edge of the bonded substrate to thereby adjust the conductance of the through-grooves for the fluid running through them.
In still another embodiment of the bonded substrate structure of the invention, the through-groove is connected to the device-housing recess formed in at least one bonded substrate, and has therein a sealing member that insulates the device-housing recess from the outside to keep it sealed.
In still another embodiment of the bonded substrate structure of the invention, the sealing member is formed by melting a sealing substance in the through-groove or on the surface opposite to the through-groove.
In still another embodiment of the bonded substrate structure of the invention, the sealing member in the through-groove is provided with a pad member having high wettability with the sealing substance, and the sealing substance is, when melted, aggregated in the area of the high-wettability pad member to block the through-groove.
In still another embodiment of the bonded substrate structure of the invention, the sealing substance is lead.
In still another embodiment of the bonded substrate structure of the invention, the sealing substance is any of a metal, an alloy or a resin.
In still another embodiment of the bonded substrate structure of the invention, only one sealing member is formed in one through-groove.
In still another embodiment of the bonded substrate structure of the invention, a plurality of sealing members are formed in one through-groove while being spaced from each other therein.
One specific embodiment of the bonded substrate structure of the invention is a micro-mirror device having a movable mirror supported by one substrate by means of beams and having, on the other substrate, an electrode for driving the movable mirror.
The second aspect of the invention is to provide a method for fabricating a bonded substrate structure with a plurality of substrates being bonded together, which comprises a step of forming, in at least one substrate to be bonded, a through-groove that runs outside from a device-housing recess formed in the substrate to thereby connect the recess to the outside, followed by bonding the substrate to another substrate, and a step of forming, in a vacuum atmosphere, an opening that passes through the surface of one bonded substrate to the device-housing recess.
The third aspect of the invention is to provide a method for fabricating a bonded substrate structure with a plurality of substrates being bonded together, which comprises a step of forming a device-housing recess in at least one substrate to be bonded, and in which the treatment for forming the device-housing recess in the step is combined with a treatment for forming a through-groove that connects the device-housing recess to the outside.
In one embodiment of the method of the invention, the step of forming the device-housing recess is for an etching treatment, and the treatment for forming the device-housing recess and the through-groove so as to make them have the same depth is effected in one and the same etching step.
The fourth aspect of the invention is to provide a method for fabricating a bonded substrate structure with a plurality of substrates being bonded together, which comprises a step of forming, in at least one substrate to be bonded, a through-groove that runs outside from a device-housing recess formed in the substrate to thereby connect the recess to the outside, followed by bonding the substrate to another substrate, a step of blocking the end of the through-groove at the edge of the bonded substrates, and a step of processing the surface of the bonded substrates.
In one embodiment of the method of the invention, the step of blocking the end of the through-groove is effected by forming a resist film.
The fifth aspect of the invention is to provide a method for fabricating a bonded substrate structure with a plurality of substrates being bonded together, which comprises a step of forming, in at least one substrate to be bonded, a through-groove that runs outside from a device-housing recess formed in the substrate to thereby connect the recess to the outside, followed by bonding the substrate to another substrate, and a step of sealing the through-groove by melting a sealing substance having been fitted to at least any of the through-groove or the surface opposite to the through-groove.
In one embodiment of the method of the invention, the sealing substance is a low-melting-point material, a pad member having high wettability with the sealing substance is formed adjacent to the through-groove, and the sealing substance is, when melted, aggregated in the area of the high-wettability pad member to block the through-groove.